Refactor RyuJIT/x86 long decomposition code

Refactor RyuJIT/x86 long decomposition code

All decomposition code is now in decompose.cpp, in a Decompose class.
Each node type that is decomposed has its own member function to do
the decomposition. Various helpers have been added to reduce code
duplication. In general, though, the code is as it was before.

Commit migrated from https://github.com/dotnet/coreclr/commit/0e5227d3924ea11f28c5f62dee681b780ee7ecaa